PHYSIOLOGY OF COLOR VISION 461 



trast, for not only is the simultaneous contrast of hues obtained most 

 strikingly when these are of equal brightness, but we constantly experi- 

 ence brightness contrast itself. Thus pieces of the same gray paper 

 placed on gray backgrounds of varying degrees of brightness do not look 

 at all alike. It is particularly at the border between the two grays that 

 contrast brightness is most evident. This subserves the function of 

 creating a sharp border between the grays, and it can be demonstrated 

 by causing strips of different gray papers to overlap one another like the 

 tiles of a roof or, still more strikingly, by rotating a disc on which when 

 spun appear three circles of different grays, each synthesized from 

 black and white. In both experiments the grays, though really perfectly 

 uniform, will appear as if shaded from their edges. 



Since we measure brightness in terms of grayness, and since it is 

 most marked at the yellow portion of the spectrum, it follows that if we 

 desire, for successful contrast effects in picture painting, to appose 

 yellows with blues or deep reds, we must employ some artificial means 

 either to increase the brightness of the blues or reds or to decrease that 

 of the yellows. This can be done by mixing the pigments with white 

 (or black), that is to say, we may alter what the artist speaks of as the 

 value of the color but which in so far as white is used for producing the 

 alteration is more correctly called the saturation. 



It may indeed be said that the object sought in mixing pigments 

 with white (i. e., changing their saturation) is to give the impression 

 that their properties of brightness have been altered. 5 When it is 

 desired to raise the brightness of a given color, we can succeed only to a 

 limited degree by using more pigment ; to obtain it further, we must, as 

 already explained, employ the property of simultaneous contrast. These 

 methods used by the artist to alter the brightness of his colors are how- 

 ever liable to have a dulling effect on the whole composition unless they 

 are used with great care and judgment. When he is compelled to lower 

 the saturation of one color he must be careful to apply those neighbor- 

 ing on it in such a manner as to give the impression that the whole of 

 that portion of the picture is of the same brightness. This he may do, 

 either by making his pigments of similar saturation or by assorting the 

 size of the colored areas, so that they appear by contrast to be of similar 

 saturation. 



It is a well-known fact that our judgment of the relative brightness 

 of colors, and to a certain extent of their hues, becomes altered when 

 the conditions of illumination are changed. A picture viewed in broad 

 daylight may create a very different impression from that which it 

 produces in dull illumination. For example, its hues may be dull and 

 muddy under the conditions of illumination that are ordinarily present 

 in a dwelling, or even in a gallery, whereas when viewed in broad day- 

 's Brightness must be distinguished from color intensity •which is purely a 

 physical property and depends upon the amplitude of the wave-lengths. 



